The invention provides oligonucleotides, method and kit for detecting Listeria monocytogenes by nucleic acid amplification and/or nucleic acid hybridization.
The genus Listeria consists of the six species L. monocytogenes, L. grayi L. innocua, L. ivanovii, L. seligeri and L. welshimeri. Among these, only strains of the species L. monocytogenes are pathogenic for humans, in particular for those with a weakened immune system and for the elderly and the newborn. The most common symptoms of listeriosis are septicemia, meningitis and miscarriages. L. monocytogenes infections are caused especially by consuming contaminated food, in particular milk products, meat, poultry and vegetables.
A large number of methods for detecting L. monocytogenes are known. Conventional detection methods for L. monocytogenes comprise preconcentrating and subsequently isolating colonies on selection media (Lovett et al., J. Food Protection 50 (1987), 188-192; McClain and Lee, J. Assoc. Off. Anal. Chem. 71 (1988), 660-664). Single colonies are examined for their morphology or for biochemical or serological properties. An analysis may take up to 6-8 days.
Since especially readily perishable food is frequently contaminated with L. monocytogenes, various high-speed methods for detecting L. monocytogenes have been developed. Such methods are based either on immunological methods or on the application of nucleic acid probes.
In this connection, detection may be carried out by direct hybridization of probes to microbe-specific DNA or RNA (see, for example, Datta, A. R. et al., Appl. Environ. Microbiol. 53 (1987), 2256-2259). The disadvantage of those methods is the low sensitivity, since at least 105-106 copies of the target nucleic acid are required. This can be compensated by combination with an amplification of the target sequence, for example using the polymerase chain reaction (PCR). A plurality of PCR methods for detecting L. monocytogenes have been described in the literature [for a review see, for example, Jones, D. D. and Bej, A. K. in xe2x80x9cPCR Technology, Current Innovationsxe2x80x9d, Griffin, H. G and Griffin, A. M., eds., (1994), 341-365]. See also U.S. Pat. Nos. 4,683,195; 4,683,202 and 4,965,188. Furthermore, the ligase chain reaction [WO publication 89/09835], xe2x80x9cself-sustained sequence replicationxe2x80x9d [EP 329,822], xe2x80x9ctranscription based amplification systemxe2x80x9d [EP 310, 229] and Qxcex2 RNA replicase system [U.S. Pat. No. 4,957,858] may be employed for the amplification of nucleic acids.
Some test kits for detection by means of antibodies are already commercially available. Most of these tests, however, display only low sensitivity and specificity.
To detect specific microorganisms by means of nucleic acid hybridization or nucleic acid amplification, microbe-specific oligonucleotides are commonly used whose base sequence is characteristic for the DNA or RNA of a specific microorganism or of a group of microorganisms. When using said microbe-specific oligonucleotides (for example as primers or probes) in connection with the methods mentioned above, hybridization to the DNA/RNA or amplification of DNA/RNA can occur under suitable reaction conditions only if the DNA/RNA of the particular microorganisms to be detected is present.
The detection methods described for L. monocytogenes are based mainly on those target genes which play a role in the pathogenicity of L. monocytogenes. It is known that some of these genes are located on the chromosome next to each other in a virulence gene cluster. Since the listeriolysin gene (hlyA) has been recognized first as to be clearly necessary for the pathogenicity of L. monocytogenes (Cossart, P. et al., Infect. Immun. 57 (1989), 3629-3636), most of the genotypic detection methods are based on this gene. The hlyA gene, however, is also found with high homology in nonpathogenic listeria (i.e. in L. seeligeri and L. ivanovii). In said detection methods, the appearance of false-positive results cannot be completely dismissed, since single point mutations in the region of the binding sites of primers or probes may already be sufficient for this.
It was possible to show that the metalloprotease gene (mpl) which is located in the genome right next to the hlyA gene is only present in L. monocytogenes, and thus not in nonpathogenic listeria (Domann, E. et al., Infect. Immun. 59 (1991), 65-72).
The suitability in principle of the DNA region flanking the hlyA gene for detecting L. monocytogenes by means of hybridization or amplification has been described (Rossen, L. et al., Int. J. Food Microbiol. 14 (1991), 145-152); however, no oligonucleotide sequences for such detection methods have been published yet.
The sequence of the L. monocytogenes mpl gene is described in the EMBL database under accession number X54619 [Domann, E. et al., Infect. Immnun. 59 (1991), 65-72]. Furthermore, parts of the sequence of the L. monocytogenes mpl gene are listed in the EMBL database under accession number X60035 [Rasmussen, O.F. et al., Infect. Immun. 59 (1991), 3945-3951].
It was an object of the present invention to develop a detection method which is suitable for routine use and in which the probability of false-positive results appearing is as low as possible for the particular user, even under very variable experimental conditions.
In particular, oligonucleotide sequences are to be provided which can be employed in a detection method for the L. monocytogenes metalloprotease gene (mpl).
These objects are achieved by providing nucleic acid molecules of the sequences
(i) 5xe2x80x2-GAA AAA GCA TTT GAA GCC AT-3xe2x80x2 or
(ii) 5xe2x80x2-GCA ACT TCC GGC TCA GC-3xe2x80x2 or
(iii) 5xe2x80x2-TCG AAA AAG CAT TTG AAG CC-3xe2x80x2 or
(iv) 5xe2x80x2-GGT CAG AGT GAA GCT CAT GT-3xe2x80x2 or
(v) 5xe2x80x2-CTI TTC ACA TGA GCT TCA CTC TGA CCR A-3xe2x80x2 or
(vi) 5xe2x80x2-CTT TTT CTT TCA CTG GGT TTC CGA CAT-3 xe2x80x2 or
(vii) 5xe2x80x2-GAT GAT TTC TTT TTC TTT CAC TGG ATT TCC AAT AT-3xe2x80x2 or
(viii) of the sequence complementary in each case to (i), (ii), (iii), (iv), (v), (vi), and (vii).
The oligonucleotides according to the invention may be defined as follows:
Oligonucleotide LM1: (sequence (i)=SEQ ID NO 1 corresponds to the position 2476 to 2495 of L. monocytogenes [according to Domann, E. et al. Infect. Immun. 59 (1991), 65-72).
Oligonucleotide LM 2: (sequence (ii)=SEQ ID NO 2) corresponds to the position 2608 to 2624 of L. monocytogenes. 
Oligonucleotide LM3: (sequence (iii)=SEQ ID NO 3) corresponds to the position 2474 to 2493 of L. monocytogenes. 
Oligonucleotide LM 4 (sequence (iv)=SEQ ID NO 4) corresponds to the position 2497 to 2516 of L. monocytogenes. 
Oligonucleotide LMR 1: (sequence (v)=SEQ ID NO 5) corresponds to the position 2495 to 2522 of L. monocytogenes. 
Oligonucleotide LMF 1: (sequence (vi)=SEQ ID NO 6) corresponds to the position 2525 to 2551 of L. monocytogenes. 
Oligonucleotide LMF 2 (sequence (vii) SEQ ID NO 7) corresponds to the position 2525 to 2559 of L. monocytogenes. 
In order to investigate the extent to which sequence variations of the mpl gene occur within the species L. monocytogenes, an internal fragment of 300 base pairs of 13 L. monocytogenes strains of various serovars (2 strains of the serovars 1/2a, 1 strain of the serovar 1/2b, 1 strain of the serovar 1/2c, 1 strain of the serovar 3a, 1 strain of the serovar 3b, 1 strain of the serovar 3c, 1 strain of the serovar 4a, 1 strain of the serovar 4a/b, 1 strain of the serovar 4b, 1 strain of the serovar 4c, 1 strain of the serovar 4d, and 1 strain of the serovar 7) was sequenced. On the basis of sequence comparisons, it was surprisingly found that the oligonucleotides LM1, LM 2, LM3, LM 4, LMF 1, LMF 2, LMR 1 and also sequences complementary thereto lead to highly specific detections in detection methods for L. monocytogenes. The preferred probes in this connection are the oligonucleotides LM4, LMR1, LMF1 and LMF2 and the sequences complementary thereto.
The invention in particular provides nucleic acid molecules which are characterized in that, with respect to at least 10 successive nucleotides of their nucleotide chain, they
(a) are identical to 10 successive nucleotides of the above nucleic acid molecules (i) to (viii) or
(b) match 9 out of 10 successive nucleotides of the above nucleic acid molecules (i) to (viii) or
(c) match 8 out of 10 successive nucleotides of the above nucleic acid molecules (i) to (viii) or
(d) are at least 90% homologous to a nucleic acid molecule according to claim 1.
The oligonucleotides of the invention may have a length usual for probes or primers, in particular for a PCR reaction, and further may have a length which can be produced by amplification, in particular by a PCR reaction, and preferably they may be from 10 to 250 bases and in particular from 15 to 30 bases in length.
They may be present in single-stranded or double-stranded form.
Thus, suitable microbe-specific oligonucleotides of the invention for detecting L. monocytogenes are nucleic acids, preferably from 10 to 250 bases and in particular from 15 to 30 bases in length, which match at least in a 10-base sequence the stated sequences LM 1, LM 2, LM 3, LM 4, LMF 1, LMF 2 and LMR 1 or the sequences complementary thereto. Relatively small deviations (1 to 2 bases) in this 10-base sequence are possible without the specificity stated in each case being lost during amplification and/or hybridization. It is known to the skilled worker that in the case of such relatively small deviations the reaction conditions have to be modified accordingly; cf., for example, T. Maniatis, Molecular Cloning, G. Sambrook and E. F. Fritsch, editors, Cold Spring Harbour [sic] Laboratory Press, 1989.
To detect L. monocytogenes, nucleic acids, preferably genomic DNA, are first released from cells contained in a sample or bacterial culture to be investigated. It is then possible, by means of nucleic acid hybridization and by using the microbe-specific oligonucleotides according to the invention as probe, directly to detect microbe-specific nucleic acid sequences in the sample to be investigated. Various methods known to the skilled worker are suitable for this purpose such as, for example, Southern blot or dot blot.
Especially because of the higher sensitivity, however, an indirect detection method is preferred in which the sought-after DNA/RNA sequences released as described above are first amplified by means of the abovementioned methods for amplifying nucleic acids, preferably PCR.
The primers employed for DNA/RNA amplification using the methods mentioned are the nucleic acid molecules according to the invention. In this connection, specific amplified molecules are formed only if L. monocytogenes DNA/RNA is present. A detection reaction (following or during the amplification reaction) using the nucleic acid molecules according to the invention as probes can increase the specificity of the detection method. In this detection reaction, oligonucleotides which are not entirely microbe-specific may likewise be used.
An alternative possibility is for the nucleic acid amplification to be carried out also in the presence of one or more not entirely specific oligonucleotides so that possibly DNA/RNA of other microorganisms not to be detected may also be amplified. An amplification method of this type is usually less specific and should therefore be safeguarded by a detection reaction (following or during the amplification reaction) using one or more of the nucleic acid molecule(s) according to the invention as probe(s).
According to the invention, it is possible to use various methods in order to detect the amplification products generated in the indirect methods. These include, inter alia, methods known per se such as visualization by means of gel electrophoresis, hybridization of probes to immobilized reaction products [coupled to nylon or nitrocellulose filters (Southern blots) or, for example, to beads or microtiter plates] and hybridization of the reaction products to immobilized probes (for example reverse dot blots or probes coupled to beads or microtiter plates). In addition, it is possible to use methods in which one or more of the nucleic acid molecules according to the invention can, as probes, qualitatively and quantitatively detect specifically forming amplification products during the PCR reaction (xe2x80x9conlinexe2x80x9d).
According to the invention, there is a large number of possibilities for the oligonucleotides according to the invention (e.g. probes and primers) to be possibly labeled or modified for the direct or indirect detection methods described. Thus, said oligonucleotides may contain, for example, radioactive, colored, fluorescent or otherwise modified or modifying groups, for example antibodies, antigens, enzymes or other substances with affinity to enzymes or enzyme complexes. Probes and primers may be either naturally occurring or synthetically produced double-stranded or single-stranded DNA or RNA or modified forms of DNA or RNA such as, for example, PNA (in these molecules the sugar units have been exchanged for amino acids or peptides). Individual or a plurality of nucleotides of the probes or primers according to the invention may have been replaced by analogous building blocks (such as, for example, nucleotides which are not naturally present in the target nucleic acid). In particular, up to 20% of at least 10 successive nucleotides of a nucleotide chain, in particular 1 or 2 nucleotides, may have been replaced by analogous building blocks known per se for probes and/or primers.
In the abovementioned indirect detection methods, detection may also involve an internally labeled amplified molecule. This may be carried out, for example, by incorporating modified nucleoside triphosphates (for example coupled to digoxigenin or fluorescein) during the amplification reaction.
The invention further provides a kit for analytical detection methods, in particular for detecting bacteria of the species Listeria monocytogenes, which kit contains one or more nucleic acid molecules according to the invention.
The nucleic acid molecules according to the invention or the appropriate kits may be used in a method for detecting the presence or absence of bacteria of the species L. monocytogenes in a sample, said method being preferably a nucleic acid hybridization and/or a nucleic acid amplification, such as a PCR. In this connection, bacteria to be detected can be distinguished from bacteria not to be detected on the basis of differences in the genomic DNA and/or RNA in at least one nucleotide position in the region of one of the nucleic acid molecules according to the invention.